This invention relates to pulse corrector circuits, and more particularly to an improved dial pulse corrector circuit in telephone signaling systems that provides corrected dial pulses with a constant percent break or duty factor.
Dial pulses in telephone systems are commonly produced by subscribed telephones at a pulse repetition frequency (PRF) in the order of 10 pulses per second with presecribed 60 millisecond break intervals and 40 millisecond make intervals. This corresponds to a 60 percent break. The PRF of dial pulses may vary somewhat, however, for equipment of different manufacturers. By way of example, equipment is available for producing dial pulses with PRFs varying from 7.5 pulses per second to 12 or 15 pulses per second. Also, dial pulses may be distorted in transmission through telephone circuitry such that the percent break of dial pulses varies as much as 90 percent during transmission. It is desirable, therefore, to employ equipment at various points of a telephone system to periodically adjust dial pulses having a range of pulse repetition frequencies to have a relatively constant percent break. The U.S. Pat. No. 3,092,691, Electronic Pulse Correction Circuit by R. T. Cleary and R. V. Burns, June 4, 1963, discloses a correction circuit for accomplishing such a function. The Cleary circuit comprises first and second series connected monostable multivibrators and a control circuit for selectively changing the value of the RC time constant of the second multivibrator at a time that is a function of the PRF of input dial pulses applied to the first multivibrator. This shortens the timing interval of the second multivibrator. In that corrector, an input break interval triggers the first multivibrator to set a time delay after which the second multivibrator is triggered to initiate a corrected break interval. The next input break interval causes the control circuit to then connect another resistor to the second multivibrator to vary the rate at which its timing capacitor charges in order to vary the time-out of the second multivibrator. This causes corrected dial pulses from the second multivibrator to have a substantially constant percent break even though the input percent break and PRF vary. In a printed circuit board version of this prior art pulse corrector, it has been found necessary to individually select timing resistors and capacitors of particular values in the second multivibrator and/or the control circuit during manufacture thereof. This hand operation requires the sole attention of an operator such that it is time consuming and expensive. In a thick film hybrid circuit version of this prior art corrector, it has been found necessary to individually functionally trim timing resistors on a substrate to precise values in order to make such pulse correctors operate properly. This trimming is required to adjust the nonlinear resistance curve of a thick film resistor so that it has a prescribed value for setting the time constant of the second multivibrator. These operations are also time consuming and expensive. U.S. Pat. No. 3,700,821, issued Oct. 24, 1952, to Bruce Russell Savage, discloses a "Digital Constant Percent Break Pulse Correcting Signal Timer" for performing a similar function.
An object of this invention is the provision of an improved dial pulse corrector circuit.